The present invention relates generally to liquid crystal devices and more specifically to schemes for driving a liquid crystal cell, such as a ferroelectric liquid crystal cell, both with and without requiring DC-balancing of the liquid crystal cell.
In the field of image generators and especially those using spatial light modulators (SLMs), it is well known that stationary and moving images, either monochrome or color, may be sampled and both color-separated and gray-scale separated pixel by pixel. These pixelated separations may be digitized, forming digitized images that correspond to the given images. These digitized images are used by devices in this field to create visual images that can be used for a direct visual display, a projected display, a printer device, or for driving other devices that use visual images as their input.
One such novel image generator is disclosed in U.S. Pat. No. 5,748,164, entitled ACTIVE MATRIX LIQUID CRYSTAL IMAGE GENERATOR, and issued May 5, 1998, which patent is incorporated herein by reference. An image generator of this type is further described in U.S. Pat. No. 5,808,800, entitled OPTICS ARRANGEMENTS INCLUDING LIGHT SOURCE ARRANGEMENTS FOR AN ACTIVE MATRIX LIQUID CRYSTAL IMAGE GENERATOR, and issued Sep. 15, 1998, which patent is also incorporated herein by reference.
As described in detail in the above recited patents, the inventions disclosed contemplate the use of a liquid crystal material, such as a ferroelectric liquid crystal (FLC) material, as a preferred light modulating medium for the spatial light modulator of the disclosed inventions. This light modulation of liquid crystal material is accomplished by establishing and maintaining electric fields across the liquid crystal material in a controlled way in order to switch the light modulating characteristics of the material. As an example, in the case of an FLC material, an electric field is established in one direction across the FLC material in order to produce a first light modulating state, for example an ON state. An electric field is established in the opposite direction across the FLC material in order to produce a second light modulating state, for example an OFF state.
Because currently available liquid crystal materials manufactured using currently available manufacturing processes are not completely insulating, and because currently available assembly processes for manufacturing liquid crystal SLMs may introduce contaminants into the SLM assembly, this formation of electric fields across the liquid crystal material may cause leakage current to flow through the liquid crystal material while the electric fields are applied to the material. If these electric fields are not balanced, the unbalanced fields (or the unbalanced leakage current) are believed to cause the degradation of the electro-optic characteristics of the liquid crystal material, thereby dramatically reducing the effectiveness and useful life of the material as a light modulating medium.
The presence of unbalanced fields across the light modulating medium tends to polarize or bias the light modulating medium if the electric fields are not balanced over time. When the electric fields are not balanced, it is believed that the net electric field in one direction causes ionic charges to migrate through the light modulating medium and build up or stick on the sides of the light modulating medium. This sticking or build up of ionic charges tends to interfere with the electric fields subsequently applied to the light modulating medium and therefore interfere with the operation of the spatial light modulator. This interference typically results in image sticking that interferes with the proper operation of the display system. For purposes of this specification, image sticking is defined as unwanted image interference during a given frame that is caused by latent electrical effects caused by previous image frames. Traditionally, this problem of image sticking is avoided or reduced by DC-balancing the driving electric field applied to the FLC material. As mentioned above, in the case of FLC materials, the materials are switched to one state (i.e. ON) by applying a particular voltage through the material (i.e. +2.5 VDC) and switched to the other state (i.e. OFF) by applying a different voltage through the material (i.e. xe2x88x922.5 VDC). Because FLC materials respond differently to positive and negative voltages, it is not a trivial matter to simply DC balance them with a single signal in situations where it is desired to vary the ratio of ON time to OFF time arbitrarily. Therefore, DC-field balancing for FLC SLMs is most often accomplished by displaying a frame of image data for a certain period of time. Then, a frame of the inverse image data is displayed (but made not visible) for an equal period of time in order to obtain an average DC field of zero for each pixel making up the SLMs.
In the case of an active matrix image generating system or display, the image produced by the SLM during the time in which the frame is inverted for purposes of DC-balancing is not typically made available to the user. If the system were viewed during the inverted time without correcting for the inversion of the image, the image would be degraded. In the case in which the image is inverted at a frequency faster than the critical flicker rate of the human eye, the overall image would be completely washed out and all of the pixels would appear to be half on. In the case in which the image is inverted at a frequency slower than the critical flicker rate of the human eye, the viewer would see the image switching between the positive image and the inverted image. Neither of these situations would provide a usable display.
In one approach to solving this problem, the light source used to illuminate the SLM is switched off or directed away from the SLM during the time when the frame is inverted. However, this approach substantially limits the brightness and efficiency of the system. In the case where the magnitude of the electric field during the DC-balancing and the time when the frame is inverted is equal to the magnitude of the electric field and the time when the frame is viewed, the light from a given light source may only be utilized a maximum of 50% of the time.
In order to overcome this problem of not being able to view the system during the DC-balancing frame inversion time, compensator cells have been proposed for SLMs. For example, U.S. Pat. No. 6,100,945, entitled COMPENSATOR ARRANGEMENTS FOR A CONTINUOUSLY VIEWABLE, DC FIELD-BALANCED, REFLECTIVE, FERROELECTRIC LIQUID CRYSTAL DISPLAY SYSTEM, and issued Aug. 8, 2000, which patent is incorporated herein by reference, discloses several approaches to providing display systems that include compensator cells. These compensator cells are intended to correct for the frame inversion during the time when the FLC pixel is being operated in its inverted state, thereby allowing the display to be substantially continuously viewable. Although these compensator cell arrangements appear to work well, they increase the complexity and cost of the display system by requiring the use of a compensator cell and in many cases other additional components.
Much of the earliest work with FLC displays also encountered the DC balance problem and a class of solutions was found. The early work dealt with passive matrix displays, because the unique properties of FLCs were expected to enable much larger displays having many more rows and columns of pixels than were then allowed using passive matrix nematic displays. There is a large amount of patent and scientific literature associated with passive matrix FLC displays. However, U.S. Pat. No. 4,709,995 issued to Kuribayashi is typical of the approach to DC balance taken in almost all such work.
In a passive matrix FLC display, the pixels are defined as the intersection of a column electrode with a row electrode. The column electrodes are formed as long, narrow, and parallel conductors that run entirely across the display with each column electrode being the width of one pixel. Likewise, the row electrodes are long, narrow, and parallel conductors that run entirely across the display in a direction perpendicular to the column electrodes with each row electrode being the height of one pixel. These electrodes typically consist of transparent Indium-Tin-Oxide, and this material is deposited directly onto the inner surfaces of two glass substrates. The column electrodes are put on one substrate, while the row electrodes are put on the second substrate. The substrates are then assembled to have the FLC layer between them.
There are no active transistors or other similar components in a passive matrix display. The FLC material comprising a pixel is forced to one of two electro-optic states (ON or OFF in the display) by the application of an electric field. In the passive matrix display, the image data are written to the display a row at a time, and all the rows are written, usually sequentially, during each image frame. Any given row is selected for writing by applying a particular voltage to the associated row electrode. Meanwhile, the image data for each pixel in the selected row are applied to each associated column electrode as a particular voltage. The difference between these two voltages provides the electric field needed to switch each specific FLC pixel. After a short time, the next row is selected and the image data are written to it with the appropriate pixel voltages applied to the columns. Typically, voltages greater than 10V magnitude are applied to the electrodes, since only such high voltages can cause the FLC to switch in the very small fraction of the frame time during which the image data are actually applied to any one row.
It is necessary to DC balance the electric field applied to any passive matrix FLC pixel, in addition to switching the pixel into the proper state. The generic method for accomplishing this in passive matrix displays is to first apply a field which would switch the pixel to the opposite state from the one that is wanted. After the false initial field, the field that will put the pixel into the desired state is then applied. This pulse-pair switching approach is accomplished by applying a succession of electrical pulses to the row and column electrodes associated with any one row during the time it is being written. The succession of pulses are arranged for each pixel so that the integral of the applied field over the row time becomes zero, and this result must be true for both the ON and the OFF states.
During most of each image frame, any given row is not selected, so that the data appearing on the column electrodes is almost always associated with the pixels of some other row. This circumstance requires that the FLC in the pixel be bistable. Bistability means that 1) the FLC must maintain the proper electro-optic state for one entire frame interval even though the electric field which selected that state is no longer present and 2) the FLC must maintain the proper electro-optic state despite the fact that voltages directed to other rows are constantly appearing on the column electrodes and these will try to perturb any given pixel from its proper state.
Much of the prior art associated with passive matrix displays, including U.S. Pat. No. 4,709,995, constitutes the disclosure of particular sequences of voltage pulses to the row and column electrodes, which sequences are especially suited to operate the pixels of passive matrix displays of various designs. All of the known methods and apparatus regarding passive matrix displays require FLC bistability. These methods and apparatus will not make a successful display if they are applied to a FLC material that is not bistable. Also, all of the passive matrix prior art concerns methods or apparatus that provide approximately DC balanced operation.
To use an FLC material that is not bistable requires that the electric field that selects the electro-optic state must be present throughout the entire frame time. A passive matrix display and the associated methods of operation cannot accomplish such continuous application of the electric field. The present invention applies to active matrix displays that maintain a selected electric field at all times. This means that the active matrix methods and apparatus of the present invention could not make use of the prior art passive matrix drive waveforms.
The present invention discloses novel methods for solving or reducing the above described image sticking problems caused by unbalanced electric fields both with and without requiring DC-balancing. These novel methods improve the effectiveness of the display system without increasing the complexity of the system, as would be the case if compensators were required.
The present invention relates generally to a method of operating a liquid crystal cell during a given period of time, the method using input image data to control how the cell is operated. The method includes applying image producing electric fields of a first magnitude to the cell during a first portion of the given period of time, the image producing electric fields depending in a predetermined way upon the input image data. The method also includes applying additional electric fields of a higher, second magnitude to the cell during a second portion of the given period of time, the image producing electric fields and the additional electric fields being such that the cumulative time integral of the electric fields that are present in one direction across the liquid crystal material is substantially equal to the cumulative time integral of the electric fields that are present in the opposite direction during the given period of time during the operation of the liquid crystal cell.
The image data may be divided into frame image data corresponding to individual frames of image data, the given period of time may be a frame time associated with one frame of image data, and the method may be a method of operating the liquid crystal cell for a plurality of frame times at a certain frame rate. The liquid crystal cell may be a ferroelectric liquid crystal cell including ferroelectric liquid crystal material. The ferroelectric liquid crystal cell may be a ferroelectric liquid crystal spatial light modulator for modulating light directed into the spatial light modulator, the ferroelectric liquid crystal material of the spatial light modulator may be divided into a plurality of individually controllable pixels, and the operation of applying image producing electric fields to the cell may includes applying image producing electric fields to each of the individually controllable pixels during the first portion of the given period of time, thereby causing the individually controllable pixels to form a desired light modulating pattern for modulating light directed into the spatial light modulator.
The spatial light modulator may be part of an overall display system that includes an illuminator for directing light into the spatial light modulator and the method may include causing the illuminator not to direct light into the spatial light modulator during the second portion of the given period of time during which the additional electric fields are being applied to the spatial light modulator.
The ferroelectric liquid crystal material may include a top and a bottom surface, the top and bottom surfaces of the liquid crystal material being approximately coplanar. The ferroelectric liquid crystal spatial light modulator may include a top electrode located adjacent to the top surface of the ferroelectric liquid crystal material and a plurality of pixel electrodes located adjacent to the bottom surface of the ferroelectric liquid crystal material, each of the plurality of pixel electrodes being associated with, and capable of controlling, one of the plurality of pixels. Applying the additional electric fields to the cell for the second portion of the given period of time may includes (i) individually setting each pixel electrode to an electric potential related in a predetermined way to at least one of the electric fields applied to that pixel during the first portion of the given period of time during which the image producing electric fields are applied to each of the individually controllable pixels and (ii) applying a constant electric potential to the top electrode of the spatial light modulator for the second portion of the given period of time.
The setting of the pixel electrodes during the second portion of the given period of time may include inverting the polarity of the fields applied to the pixels and increasing the magnitude of the electric fields. The setting of the pixel electrodes during the second portion of the given period of time may include shortening the time duration of the electric fields by an amount proportional to the increase in the magnitude of the electric fields. The second portion of the given period of time may be less than or equal to about forty-five percent of the duration of the given period of time.
The present invention also relates to a method for operating a liquid crystal display during a given period of time, the method using input image data to control how the display is operated, the display creating visible images at a viewing area. The method includes applying a first series of voltage signals to the liquid crystal display during one portion of the period of time, the first series of voltage signals being arranged to produce an image as represented by the input image data. The method also includes allowing the display to be viewed at the viewing area, while the image is being produced by the first series of voltage signals applied to the display, by allowing illumination light to be directed to the display and from the display to the viewing area. The method also includes applying a second series of voltage signals to the liquid crystal display during another portion of the period of time, the second series of voltage signals being arranged to produce an inverse image, the second series of voltage signals being related to the first series as being inverted in polarity relative to the first series, having an increased magnitude relative to the first series, and having a shorter time duration than the first series. The method also includes substantially preventing the display from being viewed at the viewing area, while the inverse image is being produced by the second series of voltage signals applied to the display, by substantially preventing illumination light from reaching the viewing area.
The display may be made viewable or substantially not viewable by controlling the light emitted from a light source operatively associated with the liquid crystal display. The image may be made viewable or substantially not viewable by selectively allowing or substantially preventing light to pass from the light source to the liquid crystal display to the viewing area. The one portion of time may be a contiguous sub-period of the given period of time. The one portion of time may be divided into a plurality of sub-periods of the given period of time. The second series of voltage signals may have a magnitude that is at least 20% greater than the magnitude of the first series of voltage signals. The second series of voltage signals may have a magnitude that is at least 50% greater than the magnitude of the first series of voltage signals. The second series of voltage signals may have a magnitude that is at least 75% greater than the magnitude of the first series of voltage signals. The second series of voltage signals may have a magnitude that is at least twice as great as the magnitude of the first series of voltage signals.
The method may further include providing separate input connections to the liquid crystal display for connection of a first external power supply for control logic within the liquid crystal display and for connection of a second external power supply for the drive voltages within the liquid crystal display that are used in applying the first and second series of voltage signals to the liquid crystal display. The second external power supply may be switched between two different magnitudes for use in generating the first and second set of voltage signals.
The present invention also relates to a liquid crystal display system with a microdisplay panel having a first voltage supply input connection operatively associated with control logic in the microdisplay panel and a second voltage supply input connection operatively associated with pixel circuitry in the microdisplay panel. The system also includes a first power supply operating at a first voltage level, the first power supply connected to the first voltage supply input connection of the microdisplay panel and a second power supply operating at a second voltage level, the second power supply connected to the second voltage supply input connection of the microdisplay panel.
The first and the second voltage levels may be different from each other.